Why does the Bible mention the mythical cockatrice?

Isaiah 11:8, "And the sucking child shall play on the hole of the asp, and the weaned child shall put his hand on the cockatrice’ den."

Isaiah 14:29, "Rejoice not thou, whole Palestina, because the rod of him that smote thee is broken: for out of the serpent’s root shall come forth a cockatrice, and his fruit shall be a fiery flying serpent."

Isaiah 59:5, "They hatch cockatrice’ eggs, and weave the spider’s web: he that eateth of their eggs dieth, and that which is crushed breaketh out into a viper."

Jer. 8:17, "For, behold, I will send serpents, cockatrices, among you, which will not be charmed, and they shall bite you, saith the LORD."

Prov. 23:32, "At the last it biteth like a serpent, and stingeth like an adder [same Hebrew word as all other verses translated as 'adder' here]."

In 1611, when the KJV was produced, the translators used "cockatrice" in part of their translations from the Hebrew. A cockatrice is a mythical creature that does not exist. It was supposedly a serpent produced from a cock's egg. So why would they use that word? They did so because they didn't know what the original Hebrew word meant, and not having a sufficient knowledge of biology they used an English word that wasn't appropriate. Today we have a much better understanding of the Hebrew, as well as biology. This is why modern translations use the words "viper" and "adder" and "poisonous snake" to translate the original Hebrew word, "tsepha".

3. Strong, J. (1996). The exhaustive concordance of the Bible : Showing every word of the text of the common English version of the canonical books, and every occurrence of each word in regular order. (electronic ed.). Ontario: Woodside Bible Fellowship.